The present invention relates to a rotating blade for a steam turbine and, more particularly, to an attachment arrangement for attaching a blade of a steam turbine to a rotor that minimizes local and average stresses.
The steam flow path of a steam turbine is formed by a stationary cylinder and a rotor. A number of stationary vanes are attached to the cylinder in a circumferential array and extend inward into the steam flow path. Similarly, a number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades form a stage. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. The blade airfoils extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached to it.
The blade airfoils extend from a blade root used to secure the blade to the rotor. Conventionally, this is accomplished by imparting a fir tree shape to the root by forming approximately axially extending alternating tangs and grooves along the sides of the blade root. Slots having mating tangs and grooves are formed in the rotor disc. When the blade root is slid into the disc slot, the centrifugal load on the blade, which is very high due to the high rotational speed of the rotor—typically 3600 rpm for a steam turbine employed in electrical power generation, is distributed along portions of the tangs over which the root and disc are in contact. Because of the high centrifugal loading, the stresses in the blade root and disc slot are very high. It is desirable, therefore, to minimize the stress concentrations formed by the tangs and grooves and maximize the bearing areas over which the contact forces between the blade root and disc slot occur. This is especially desirable in the latter rows of a low pressure steam turbine due to the large size and weight of the blades in these rows and the presence of stress corrosion due to moisture in the steam flow. The latter stages experience higher local stresses that may lead to lower fatigue life of the rotor and the rotating blades. There is also an increasing demand for longer rotating blades, which requires the rotor and blades to operate under even higher loads.
In addition to the steady centrifugal loading, the blades are also subject to vibration.
It is therefore desirable to provide a rotor and blade attachment configuration that has centrifugal load carrying capability, reduced local stresses on the rotor (wheel) and the fillets of the rotating blades, while maintaining average and shear stresses low.